We present three cases in whom mental disorders appeared after orthognathic surgery.The first case was a 35-year-old female who had a history of depression. After the operation, a manic state appeared and she was diagnosed with bipolar disorder. Her medicine was changed from an antidepressant to a mood stabilizer, after which her mental condition stabilized.The second case was a 34-year-old male. He could not accept the appearance of his postoperative face, and it took about three months for him to finally accept his new appearance.The final case was a 37-year-old female. She could not accept the appearance of her postoperative face because it was not what she had expected. She began to complain about her face and became depressive, and was finally diagnosed with major depressive disorder. After administration of an antidepressant, her mental condition gradually improved.We must take into consideration the possibility that patients will develop mental disorders after orthognathic surgery, and adequate explanation and patient assessment before the operation are therefore important.